TST: clean up stats.kde test case, it was too heavily parametrized
TST: clean up stats.kde test case, it was too heavily parametrized

Testing all combinations of dtypes for 4 different parameters
makes little sense, this results in dozen of test failures that
are all the same. This reduces it to two test failures for `float128`
with the NEP 50 branch of numpy.

rgommers/scipy,Stefan-Endres/scipy,endolith/scipy,Stefan-Endres/scipy,perimosocordiae/scipy,rgommers/scipy,scipy/scipy,mdhaber/scipy,WarrenWeckesser/scipy,ilayn/scipy,ilayn/scipy,andyfaff/scipy,scipy/scipy,endolith/scipy,perimosocordiae/scipy,rgommers/scipy,endolith/scipy,scipy/scipy,andyfaff/scipy,endolith/scipy,endolith/scipy,mdhaber/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,ilayn/scipy,ilayn/scipy,andyfaff/scipy,mdhaber/scipy,ilayn/scipy,anntzer/scipy,anntzer/scipy,Stefan-Endres/scipy,anntzer/scipy,andyfaff/scipy,mdhaber/scipy,mdhaber/scipy,anntzer/scipy,mdhaber/scipy,perimosocordiae/scipy,WarrenWeckesser/scipy,WarrenWeckesser/scipy,rgommers/scipy,Stefan-Endres/scipy,scipy/scipy,Stefan-Endres/scipy,anntzer/scipy,andyfaff/scipy,anntzer/scipy,endolith/scipy,perimosocordiae/scipy,WarrenWeckesser/scipy,ilayn/scipy,andyfaff/scipy,perimosocordiae/scipy,scipy/scipy,scipy/scipy,perimosocordiae/scipy,rgommers/scipy,WarrenWeckesser/scipy